1. Field of the Invention
This invention relates to an outboard motor engine speed control system.
2. Description of the Related Art
When a boat is driven by two or more outboard motors mounted side by side, variance in engine speed among the outboard motors causes differences in thrust that degrade the boat's straight-forwarding (course-holding) ability. Operators have therefore had to synchronize (make equal) the speeds of the internal combustion engines mounted on the outboard motors by regulating them individually. This is a tedious and complex operation. To overcome this inconvenience, outboard motor speed control systems have been developed that detect the engine speeds of the individual outboard motors to determine the outboard motor operating at the highest engine speed and synchronize the engine speeds of the other outboard motor(s) with the highest one.
Further, Japanese Laid-Open Patent Application No. Hei 8(1996)-303269 teaches a technique for the motors whose engines are switched between full-cylinder operation (during which all of the cylinders are supplied with fuel to be operative) and cut-off cylinder operation (during which the fuel supply to some of the engine cylinders are cut off or stopped to be non-operative). In the technique, the timing of implementing the cut-off cylinder operation is synchronized among the motors so that at the time switchover between the cut-off cylinder operation and full-cylinder operation, no variance in thrust arises among the outboard motors.
Another widely adopted practice is to utilize outboard motor speed differentiation positively for improving boat turning performance.
When, as in the prior art, the engine speeds of multiple outboard motors are detected and all of the outboard motor engine speeds are synchronized with the highest speed, all outboard motors come to be synchronized on the highest thrust. This degrades the feel of operation because it gives the operator an unnatural feeling when low-speed is required, such as during trolling.
Further, in order to utilize outboard motor speed differentiation positively for improving boat turning performance, it is necessary for the operator to manually disable engine speed synchronization control. As this complicates operation, there is room for improvement. It should also be noted that the technique taught by the foregoing patent application does not offer a solution for this issue because it takes into consideration only variance in thrust occurring at switchover between cut-off cylinder operation and full-cylinder operation.